This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-134069, filed May 14, 1999; No. 11-222476, filed Aug. 5, 1999; No. 11-228519, Aug. 12, 1999; and No. 11-228520, filed Aug. 12, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a reverse biasing apparatus for a solar battery module, particularly, for an amorphous thin-film solar battery module. More specifically, the present invention relates to an apparatus which applies a reverse bias voltage lower than a breakdown voltage between a substrate side electrode and a back electrode in a thin-film solar battery module, which sandwich a photovoltaic semiconductor layer that contributes to power generation, thereby removing short-circuited portions or oxidizing them to make into insulators by means of Joule heat.
FIGS. 1A and 1B show a structure of a thin-film solar battery module 10. As shown in those figures, a first electrode layer (transparent electrode) 2 of a transparent conductive oxide, such as SnO2, is formed on an insulating substrate 1 such as glass, and is separated into strings by laser scribing. A photovoltaic semiconductor layer 3 having a stacked structure of, for example, a p-type a-Si layer, an i-type a-Si layer and an n-type a-Si layer is formed on the transparent electrode 2 and is separated into strings by laser scribing at positions different from the scribe lines of the transparent electrode 2. A second electrode layer (back electrode) 4 of metal is formed on the semiconductor layer 3, and is separated into strings by laser scribing at positions different from the scribe lines of the semiconductor layer 3. Shifting the scribe lines of each layer permits the end portion of the second electrode layer 4 of a given solar cell to be connected to the end portion of the first electrode layer 2 of an adjacent solar cell through the semiconductor scribe line, so that multiple solar cells are connected in series.
If a pin-hole is formed in the photovoltaic semiconductor layer in the individual solar cells that constitute the solar battery module during fabrication, the first electrode layer and the second electrode layer of that solar cell may be short-circuited to each other. Because the short-circuited solar cell does not contribute to power generation any more, the power generation characteristic of the solar battery is deteriorated. The power generation characteristic is improved by performing a process of applying a reverse bias voltage to the solar cells to eliminate the short-circuited portions (reverse biasing process).
Referring now to FIGS. 1A and 1B, a description will be given of the case where a short-circuited portion S is produced in a photovoltaic semiconductor layer 3b of a solar cell 5b. In this case, a pair of probes 6a and 6b are brought into contact with the second electrode layer 4b of the solar cell 5b and the second electrode layer 4c of the adjacent solar cell 5c (the second electrode layer 4c is connected in series to the first electrode layer 2b of the solar cell 5b), respectively, and apply a reverse bias voltage lower than the breakdown voltage between the first electrode layer 2b and the second electrode layer 4b which sandwich the photovoltaic semiconductor layer 3b that contributes to power generation. As the reverse bias voltage is applied, a current concentrates on the short-circuited portion, thereby generating Joule heat, and therefore, the metal material forming the second electrode layer is broken up or is oxidized into an insulating film at the short-circuited portion S. This eliminates the short-circuited portions, so that deterioration of the power generation characteristic at the time of operation can be suppressed.
A plurality of pin-holes are, however, randomly produced in each solar cell. In the case where the reverse bias voltage is applied to such a solar cell having pin-holes with a pair of probes in contact thereto, if there is a short-circuited portion S remote from the probes in the longitudinal direction of the solar cell, a voltage drop cannot be neglected. This raises various problems. In the case where the distance from the probes to a short-circuited portion is short, a sufficient current flows through the short-circuited portion so that the short-circuited portion can be removed because the short-circuited portion is broken up or oxidized as described above. On the other hand, in the case where the distance from the probes to a short-circuited portion is long, a current flows through the short-circuited portion becomes insufficient so that the short-circuited portion cannot be removed because the short-circuited portion cannot be broken up or oxidized. If the reverse bias voltage is increased to surely remove the short-circuited portion remote from the probes, a large current flows through a short-circuited portion located near the probes, generating a large amount of heat, which may make the pin-holes larger. Also, a voltage higher than the breakdown voltage may be applied to normal device regions, thus damaging the normal regions.
The present inventors disclose in Jpn. Pat. Appln. KOKAI Publication No. 10-4202 a reverse biasing apparatus which has a pair of probe lines, each probe line having a plurality of point-contact probes per string or having one or a plurality of line-contact probes or surface-contact probes per string along the longitudinal direction of solar cells. The reverse biasing apparatus can make the distance between the probes and any short-circuited portions short enough to make the voltage drop negligible. Therefore, the apparatus can overcome problems that some short-circuited portions cannot be eliminated or normal regions are damaged.
In the conventional reverse biasing apparatus, a pair of probes (or a pair of probe lines) is moved downward to be in contact with the second electrode layers of a pair of solar cells, and a reverse biasing process is carried out, and then a pair of probes is moved upward and is moved to the position on the subsequent pair of solar cells. These operations are repeated corresponding to the number of strings of solar cells. In this case, since a plurality of point-contact probes are provided per string or one or a plurality of line-contact probes or surface-contact probes are provided per string in the longitudinal direction of solar cells, difference in height between the probes and the solar cells may inevitably be produced depending on the locations of the probes. In order to prevent solar cells from suffering mechanical damages caused by a large stress produced locally, the probes should be slowly moved downward. Therefore, it takes a long time to complete the reverse biasing process for all of the several tens of strings of solar cells, thus lowering the production efficiency of solar battery module. In addition, as the probes are moved up and down a number of times, the possibility of wear-originated machine failures increases.
Conventionally, the reverse biasing process is carried out by applying a dc reverse bias voltage or by applying a reverse bias voltage having a pulse-like rectangular waveform between a pair of probes 6a and 6b. 
However, a solar battery is equivalent to a diode. When the reverse bias voltage is applied to the first electrode layer 2 and the second electrode layer 4, therefore, the solar cell 5 that comprises the first electrode layer 2, the photovoltaic semiconductor layer 3 and the second electrode layer 4 functions as a capacitor, so that charges are likely to be stored even after the voltage application is stopped. It has been found that a voltage induced by the stored charges may damage a weak portion of the photovoltaic semiconductor layer 3 other than the short-circuited portion. It has also become apparent that storage of charges by application of the reverse bias voltage occurs very easily and the adverse effect of the stored charges is greater than expected.
Accordingly, it is an object of the present invention to provide a reverse biasing apparatus capable of efficiently performing a reverse biasing process on a solar battery module having integrated multiple strings of solar cells.
It is another object of the present invention to provide a reverse biasing apparatus which prevents other portions than short-circuited portions from being damaged by suppressing storage of charges between electrodes as much as possible at the time of performing a reverse biasing process on a solar battery module.
According to one aspect of the present invention, there is provided a reverse biasing apparatus for removing short-circuited portions in a solar battery module having a plurality of solar cells each including a first electrode layer, a photovoltaic semiconductor layer and a second electrode layer, all formed on a substrate, by applying a reverse bias voltage to individual solar cells, which apparatus comprises probes to be in contact with the second electrode layers of adjacent three or more solar cells; an actuator for actuating the probes up and down; and a relay switch for selecting, from the probes, a pair of probes to be in contact with the second electrode layers of an arbitrary pair of adjacent solar cells.
The apparatus of the present invention should preferably have probes to be in contact with the second electrode layers of five to ten adjacent solar cells. It is preferable that a plurality of point-contact probes should be provided per a single string of solar cell along a longitudinal direction thereof.
The apparatus of the present invention may further comprise a function generator for supplying a reverse bias voltage having a periodically changing waveform to the solar cells through the pair of probes, and a control unit for controlling an application time for the reverse bias voltage applied by the function generator. An amplifier incorporating a current limiter may be provided between the function generator and the probes.
With the above structure, a reverse biasing process is carried out by causing the function generator to supply a reverse bias voltage having a periodically changing waveform and a predetermined peak value to the probes for a predetermined period of time and then supply a reverse bias voltage having a periodically changing waveform and a peak value higher than the predetermined peak value to the probes for another predetermined period of time. In this case, it is preferable that the control unit should control a time of supplying a reverse bias voltage having a periodically changing waveform and a predetermined peak value to 0.2 second or less. It is also preferable that an initial reverse bias voltage be supplied to the probes from the function generator with a peak value of 2V or lower. It is preferable that the current limiter incorporated in the amplifier should perform control in such a way that an absolute value of a maximum current at a time of applying the reverse bias voltage is equal to or smaller than twice the short-circuit current when sunlight of AM 1.5 is irradiated on the solar cells.
According to the present invention, the reverse bias voltage in use has a waveform of a sinusoidal wave, a half sinusoidal wave, a sawtooth wave or a rectangular wave. The reverse bias voltage may essentially include a reverse bias component and partially include a forward bias component. The reverse bias voltage preferably has a frequency of 20 to 1000 Hz, and more preferably has a frequency of 50 to 120 Hz. The function generator may supply to the probes a forward bias voltage between a time of supplying a reverse bias voltage having a predetermined peak value and a time of supplying a reverse bias voltage whose peak value is higher than the predetermined peak value.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.